Existing dissolved air flotation (DAF) wastewater treatment technology typically provides low efficiency removal of soluble biological oxygen demand (BOD) even when operated with high efficiency chemical coagulation and flocculation. In many situations of industrial wastewater pretreatment, a wastewater treatment facility operator is required to reduce BOD concentrations below 300 mg/L, and even below 250 mg/L, which cannot be consistently achieved without also achieving consistent partial soluble BOD removal. However, to obtain the required soluble BOD removal necessary to meet the total BOD pretreatment permit limit, some controlled level of biological treatment is also required to remove soluble BOD by converting soluble carbonaceous pollutants that cannot be removed by conventional DAF treatment operated with chemical coagulation and flocculation, into insoluble biomass solids that can be removed by chemical treatment and flotation in a DAF cell.
A conventional flow equalization basin which is used to provide hydraulic flow equalization upstream from a DAF cell cannot be effectively used to remove sufficient soluble BOD because the growth of biomass solids necessary to achieve consistent and adequate soluble BOD removal must be achieved by the recycle of solids skimmed from the downstream DAF. Because these DAF cell skimmings solids typically contain a high concentration of oil and grease (O&G) that is removed by flotation, these solids cannot be recycled back into the upstream flow equalization basin (FEB) without causing the buildup of excessive O&G in the FEB tank which would cause the DAF pretreatment process to fail. In addition, the high concentration of raw wastewater solids in the FEB influent wastewater makes it difficult to also develop an adequate concentration of biomass solids. The method of treatment typically used to avoid these O&G build ups, and, excessive influent solids loading problems in the FEB tank is to use a first stage DAF cell operated for O&G and total suspended solids (TSS) removal upstream from the FEB followed by a second stage DAF cell operated for BOD and TSS removal downstream from the FEB. Chemical treatment may be used in the first stage DAF cell to assist in O&G and solids removal upstream of the FEB tank.
The installation cost for such a two stage DAF pretreatment system with an intermediate FEB is typically prohibitive for many industries because two DAF cells and a large FEB tank are required to provide capability to consistently achieve the soluble and total BOD removal necessary to meet BOD pretreatment limits; and ammonia nitrogen, total Kjeldahl nitrogen (TKN), and total nitrogen removal to meet ammonia nitrogen, TKN, or total nitrogen pretreatment limits.